X-ray mask and method for providing same

ABSTRACT

The present invention describes a method for fabricating an x-ray mask tool which can achieve pattern features having lateral dimension of less than 1 micron. The process uses a thin photoresist and a standard lithographic mask to transfer an trace image pattern in the surface of a silicon wafer by exposing and developing the resist. The exposed portion of the silicon substrate is then anisotropically etched to provide an etched image of the trace image pattern consisting of a series of channels in the silicon having a high depth-to-width aspect ratio. These channels are then filled by depositing a metal such as gold to provide an inverse image of the trace image and thereby providing a robust x-ray mask tool.

The United States Government has rights in this invention pursuant toContract No. DE-AC04-94AL85000 between the United States Department ofEnergy and Sandia Corporation, for the operation of the Sandia NationalLaboratories.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to contact lithographic masks havingsubmicron features, and to a method for producing such masks. Thesetools are useful in the preparation of plating molds for fabricatingmetal microparts and are particularly useful for providing molds havinglateral feature dimensions on the order of tenths to hundredths ofmicrons while also having feature depth dimensions on the order of tento hundred times those dimensions.

2. Description of Related Art

A variety of methods are presently known for making microparts. U.S.Pat. No. 5,256,360 to Li, teaches the use of a precisely controlledmicro-electrode discharge machine (EDM) to make the micro-filter moldand suggests the use of laser-beam micro-machining, or electron-beammicro-machining, as suitable alternative processes. However, Li ('360),also teaches that molds made using conventional integrated circuits (IC)processing and lithographic processes in silicon tend to incorporatehigh internal strain, are prone to damage, are expensive to produce, andthus not economical to manufacture.

U.S. Pat. No. 5,501,893 to Laermer, et al. describes a lithographictechnique for etching silicon, generally referred to as “anisotropicetching,” where it is possible to achieve deeply-extending trencheswhile simultaneously providing side walls which are as nearly paralleland vertical as desired. In order to achieve these geometries it isnecessary to allow etching to progress only on the bottom of the etchedtrench in the silicon substrate and not on the walls of the trench. Inparticular, Laermer ('893) teaches a two stage process for alternatelyetching an exposed silicon surface in a reactive ion plasma followed bycoating the etched surfaces with a thin polymerized layer, wherein thepolymer coating serves to protect the wall surfaces of the trench fromaction of the plasma since these surfaces are not directly face theincoming flux of plasma ions. However, the polymer layer applied to the“floor” surface of the trench quickly breaks down in the presence of theion bombardment since this surface directly faces the incoming ions. Thepolymer layer, therefore, forms a very effective etching “stop” on thoseedges or surfaces not directly in the path of the ion flux allowing fordirectional etching.

The process continues in this manner, alternating etching steps withcoating steps, until the predetermined etching depth of the structuresin the silicon substrate is reached.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a processfor fabricating highly accurate, three dimensional x-ray masking tools.

It is another object to provide an x-ray mask comprising a siliconsubstrate having a foil-like metal pattern embedded into the thicknessof the substrate.

Yet another object of the invention is to provide an x-ray mask havingan embedded metal pattern whose thickness is sufficient to attenuatevirtually all x-ray radiation having an energy at or below 10 KeV whichstrikes the pattern in a direction parallel to the metal thickness.

It is another object of the invention to provide an x-ray mask whereinthe embedded pattern comprises a plurality of structural elementsexhibiting features having lateral dimensions of much less than 1micron.

Still another object of the invention is to provide an x-ray maskwherein the features include both the structural elements comprising thepattern, and the separation distances between those elements.

Yet another object of the invention is to provide an embedded patternhaving features exhibiting a height-to-width aspect ratio of greaterthan about 30-to -1.

Another object of the invention is to provide a robust x-ray mask toolwhich is capable of withstanding repeated handling and very longexposure to high-dose x-ray radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the first step in an embodiment of the present methodwherein a silicon substrate wafer, or the like, is provided.

FIG. 1B illustrates the application of a photoresist film onto a topsurface of the substrate wafer.

FIG. 1C shows the placement of a negative(positive) trace image of adesired pattern on the photoresist.

FIG. 1D a shows the exposure of the uncovered portions of thephotoresist film to a source of radiation in order to transfer apositive(negative) image of the trace image into the photoresist.

FIG. 1E shows the silicon substrate after portions of the photoresistlayer are developed and removed thereby exposing portions of siliconsubstrate.

FIG. 2A illustrates the silicon substrate covered by the developedphotoresist in which exposed portions of the substrate are subjected toan etching plasma.

FIG. 2B illustrates the silicon substrate covered by the developedphotoresist after the exposed portions of the substrate have been deeplyetched.

FIG. 3A shows the etched silicon substrate, wherein the remainingphotoresist is removed.

FIG. 3B shows the silicon substrate wherein layers of chromium and ofgold are vapor deposited such that the entire top surface of thesubstrate, those portions which are etched and those which are not, iscoated with a thin layer of these metals.

FIG. 3C shows the metal coated silicon substrate having a second thickerlayer of gold is deposited such that the etched portions on thesubstrate are completely filled onto the thin chrome/gold layer of FIG.3B.

FIG. 4 shows the removal of the excess thick gold layer from the topsurface of the silicon substrate by planarizing that surface until thesilicon substrate is again exposed.

FIG. 5 shows the step in which the back surface across a region beneaththe gold pattern is thinned by a blanket etch process until thethickness of the wafer beneath the gold pattern is reduced to less thanabout 100 microns and thereby providing the final mask embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for fabricating a robust x-raymask tool. In particular, the present invention provides a process forfabricating an x-ray mask tool capable of replicating features havinglateral dimension of less then 1 micron. Such a mask has great utilityfor production of metal and ceramic microparts by the well-known LIGAprocesses since current technology is limited to providing part moldshaving lateral features sizes greater then 1 micron.

Lithographic masks are available with features less then 1 micron andare fabricated typically in 1000 Å chromium supported on glass slips.However, in order for a mask to effectively stop the high energy, highfluence synchrotron radiation used to prepare molds for LIGA microparts,the image-stop layer, opaque to x-rays, would need to be several micronsthick. Typically, LIGA x-ray masks have a gold image-stop layer at least8 microns thick. Masks this thick are very difficult to produce byconventional means if this layer includes features of less than 1 micronacross.

The instant invention employs a combination of processes to produce apatterned mask which overcomes this limitation. A mask pattern isreplicated in a thin layer of photoresist applied to a siliconsubstrate, developed to expose the pattern on the surface of thesilicon, the exposed areas deeply etched by a reactive plasma techniqueto provide a series of trenches on the silicon surface, the trenchesfilled by plating a metal opaque to high energy x-rays, the platedsurface planarized, and the substrate thinned to provide the desiredmask.

General Description

This invention describes a lithographic mask having x-ray attenuatingstructures embedded in an essentially x-ray transparent support media.Furthermore, the invention describes a lithographic mask having featureswhich can have lateral dimensions much smaller than 1 micron across,whether those features are the x-ray blocking structures themselves orthe separation spaces between such structures.

The process begins with a standard silicon substrate. A layer of apolymer photoresist is placed onto a top surface of substrate such thatthe layer is no more than 1-2 microns thick. Any technique for applyingsuch layers may be used, including dipping, spraying, spinning or vapordepositing, and either organic or inorganic resists may be used. Themethod of application and composition of the resist is not criticalexcept for the need for providing a coating layer of less than 2microns.

The resist layer is baked, or otherwise cured, and the desired imagepattern rendered onto the layer top surface by using any of a number ofconventional lithographic processes, such as by a direct contacttransmission mask. It is also possible to create the desired pattern byimaging the reflection of a non-contact mask through camera optics ontothe resist surface, or by directly “writing” the image by using aprogrammable e-beam writer. Important to the proper operation of theinvention is the ability of the exposing “light” used to penetrate thefull depth the resist since it is known that as “light” wavelengthsdecrease toward the hard UV (<190 nm) their penetrating power issignificantly reduced necessitating thinner resist layers. Being able tofully penetrate the resist layer will allow the user to achieve the verysmall lateral dimensions desired. Use of a thin resist layer and abroadband light source helps to satisfy this requirement.

Since the resist coating will act as an etchant barrier duringsubsequent processing, the amount of protection needed will bedetermined by the processing necessary to provide the desired structure.Different combinations of resist compounds provide additional options.In the present case a thin polymer resist is placed directly onto asilicon substrate, cured, masked and exposed to broadband light. Such astructure can provide about a 50-to-1 processing-protection ratio; asufficiently robust etchant barrier to allow etching deep, narrow,channel structures in the silicon substrate. A composite resistcomprising a thin layer of conventional polymer resist may be appliedover a thin silicon dioxide layer grown onto the silicon substrate,where UV light is used to create the image pattern. Such resists permitdirect transfer of the image into a silicon dioxide (glass) “hard”resist which provides a processing protection ratio of 200-to-1 which isabout equivalent to the former resist barrier since the glass resistlayer is much thinner, typically about 1000 Å.

After rendering the image of the mask into the resist, the resist layeris chemically “developed” and the exposed areas of the resist eitherremoved or retained, depending upon the specific resist chemistry used.

Following the development of the resist, the patterned substrate isexposed to a series of anisotropic reactive etching steps such as thoseset forth in the so-called BOSCH process described in U.S. Pat. No.5,501,893, herein incorporated by reference in its entirety. In this, orsimilar anisotropic processes, the top surface of the silicon substrateis protected by the retained resist layer. This first etching step isfollowed by a first polymerization step which coats the walls, edges andbases of the etched recesses in the silicon substrate. Plasma reactorparameters and etching times are adjusted and limited to avoid excessivedamage to the resist layer and the process proceeds in this manner,alternating between etching and coating steps, until a etch depth ofbetween 10 to 30 microns is achieved. In particular, in order for anmask to effectively stop the high energy synchrotron radiation used toprepare molds for LIGA microparts, a thickness of at least 8 microns ofgold is necessary. Etch depths of at least this dimension are thereforecritical to the success of this invention.

After etching the silicon substrate, the remaining resist is strippedaway and the substrate cleaned, after which a “seed” layer of 0.025microns of chromium followed by 0.08 microns of gold is vapor depositedonto the entire surface. Alternately, this layer may be omitted if thesubstrate used is a doped, highly conductive, form of silicon.

Where the more conventional undoped silicon substrate is used, a second,thicker gold layer is deposited over the “seed” layer so as tocompletely fill and cover the etched recesses. Coating is typically doneby electroplating or by electroless deposition onto the “seed” layer butmay be done by any method providing the applied layer is uniform incomposition and structure and provides a continuous, condensed layer.The thick x-ray blocking layer may be laid down, for instance, bycontinuing the vapor deposition of the “seed” layer, by plasma spraying,or by epitaxy deposition. Time and cost, however, favor a platingprocess.

Once plated, the incipient mask is planarized by lapping the top surfaceof the substrate in order to remove the metal layers from this surfaceleaving the surface flat, and essentially free of the plated metal. Whatremains is a silicon substrate with a fine metal structure embedded intothe thickness of the substrate forming an imaging pattern comprising agold (or other similar x-ray opaque material) “ribbon” structuresextending to a depth of 10 microns or more wherein the widths of thestructures may be less than 1 micron is provided

In a final step, the back surface of the silicon substrate, the sideopposite the planarized surface, is etched away in a region underneaththe plated gold pattern to a depth sufficient to reduce the totalthickness of the silicon in this region to below about 100 microns. Thisis done because it is known that x-ray radiation at energy levels ofabout 10 KeV is not significantly attenuated by passing through siliconof these thicknesses.

Specific Description

An embodiment of the steps of the invention are described with referenceto FIGS. 1 through 5.

As required, detailed embodiments of the present invention are disclosedherein. However, it is to be understood that the disclosed embodimentsare merely exemplary of the present invention which may be embodied invarious systems. Therefore, specific details disclosed herein are not tobe interpreted as limiting, but rather as a basis for the claims and asa representative basis for teaching one skilled in the art to variouslypractice the present invention.

Referring to FIG. 1A, the process begins with a silicon substrate orwafer 10. This substrate can, generally, having any useful shape andthickness but should of necessity be a thin wafer having parallel topand bottom surfaces 11 and 12. In particular the present invention ismost easily implemented by using an industry standard 100 mmØ×0.67 mmthick wafer. In FIG. 1B a liquid photoresist film 20 (herein SRP 3612Novolak) is applied by spin coating to a thickness of about 1.8 micronsor less, and then baked at a temperature of 95° C. for about 90 secondsin order to at least partially cure the resist layer. The particularresist thickness is chosen so as to balance the need for providing athick enough layer to protect the unexposed portions of the siliconsubstrate from the effects of the later ion etch phase against thedesire to fully expose the full thickness of the resist during the lightexposure phase.

In a next step, shown in FIG. 1C, a standard direct-contact lithographicmask 13, herein embodying a negative trace image of the desired pattern,is placed directly on the surface of the of resist layer 20 (FIG. 1Cintentionally shows mask 13 above this surface for clarity sake only).In FIG. 1D the exposed portions 14, of the resist layer 20 are subjectedto a source (not shown) of broadband light, 15. Mask 13, is itselfformed by depositing a 1000Å-5000Å thick layer of chromium, or similar,into a glass support slip and comprises a plurality of lines and otherstructures and features, and separations between features, some of whichhave minimum lateral dimensions (dimensions in the plane of the mask,perpendicular to separate pattern features) of less than 1 micron. Theresist exposure source used herein was a high pressure mercury-vaporlamp emitting light over a spectral range of about 365nm to 450nm andproviding a dose of approximately 80 millijoules/cm² measured at awavelength of 365 nm.

In the next step in the process, illustrated in FIG. 1E, the photoresistis chemically “developed” and the exposed portions, 14, of photoresistlayer 20 are removed. What remains are the unexposed portions, 22, ofthe resist in an inverse image of the mask pattern wherein this inverseimage comprises “clear” areas 23 of exposed silicon. Again, this step isperformed using standard and well-known lithographic processes.

It should be noted that the choice of a positive or negative image maskdepends largely on the nature of the photoresist used, i.e., dependingupon whether or not the exposed portion of the photoresist is removed orleft intact after the resist has been developed. Either approach ispossible, although, depending on the nature of the desired pattern, oneis usually more preferred than the other.

After cleaning and drying, the patterned substrate 10 is subjected to ananisotropic reactive plasma etching process, shown in FIG. 2A, such asthe BOSCH or other similar etch-and-coat technique, wherein the exposedareas 23 of the silicon substrate 10 are etch to a depth d which issubstantially greater than the width w of etched channels 25. This stepprovides the very high aspect ratio etched pattern shown in FIGS. 2B. Asnoted supra. the BOSCH process is a two step etch-and-coat processwherein the intervening coating step comprises coating the exposedsilicon with a thin layer of a polymer film 24 which protects the wallsand edges of the etched channel but is quickly destroyed on thosesurfaces which directly face the bombardment of the reactive plasma 26shown in FIG. 2A. This action has the effect of etching channels ortrenches in the exposed silicon which have a substantially uniform widthand substantially parallel walls. The process continues until thedesired etch depth d has been achieved. In the case of the presentinvention the desired depth was 30 microns but any depth, which achievesthe stated intent of creating an x-ray blocking mask, is possible.

After etching the silicon wafer 10 to the desired depth, the remainingresist layer 22 is removed and the part cleaned leaving substrate 10with a pattern of etched surfaces 27 across top surface 11 of the wafer.The entire surface is subsequently covered with a thin electricallyconductive metal film 30, as shown in FIG. 3B, in preparation for a muchheavier coating. The chosen process for applying the first thin coatingof FIG. 3B is a thermal evaporation or particle vapor deposition (PVD)process, although any other coating process which would provide a thin,continuous layer of conductive material would be equally effective.However, any such processes must be able to coat both the walls 28 andthe bases 29 of the etched channels 25. Such methods could include, butare not limited to, sputtering and chemical vapor deposition or sprayingcoating methods, and only require that the coating process provide acontinuous, adherent, and conductive layer.

As disclosed herein, the film 30 is about a 250 Å (0.025 microns) layerof chromium with an overlaying layer of about 800 Å (0.08 microns) ofgold. Any similar metal or combination of metals would be usefulincluding most of the metals in the Transition series of metal listed inNew IUPAC Group Numbers 4-12 of the Period Table of elements, alloysthereof, and certain of the metals of Groups 13 and 14, such as aluminumand tin.

Film 30 is necessary to enable adherence of a final, thicker metal layer31 which is deposited in a subsequent step, shown in FIG. 3C. In thepresent invention, layer 31 is also gold but, as before, could be anysimilar metal selected from the list supplied above, providing that theetch depth d of the mask is adjusted to provide for a sufficiently thicklayer of metal to effectively block or substantially attenuate theaforementioned synchrotron flux while remaining below a 100 micronsthickness limit known to be about the limit at which silicon is nolonger “transparent” to such radiation but will itself begin toattenuate the x-ray beam and thus will begin to degrade to transmissionand resolving power of the x-ray mask.

Following the final step of depositing the thick x-ray blocking layer31, the mask assembly is planarized, as shown in FIG. 4, to remove metalfrom across top surface 11 of supporting silicon substrate 10, and toprovide a planarized surface 32. Planarizing is typically performed bylapping the top surface until the surface of the silicon is reachedleaving only the embedded metal pattern 33 exposed. This is done toremove the “overburden” x-ray blocking metal layer on the top surface ofthe substrate leaving only the metal deposited in etched channels 25.Planarized surface 32 is also intended to be as flat and smooth aspossible since it is the surface which will lay against the surface ofthe material onto which the synchrotron radiation is to be illuminated.

A final thinning step, illustrated in FIG. 5, is intended to reduce thethickness of silicon substrate 10 across a region 34 beneath theembedded metal pattern 33. Thinning is performed on the back side 12 ofwafer 10 using a standard blanket etching techniques until the thicknessof silicon everywhere underneath region 34 of the metal pattern 33 isreduced to about less than 100 microns. As explained above it is knownthat silicon is transparent or nearly transparent to synchrotronradiation of 10 KeV at thicknesses below about 100 microns.

Finally, because a plurality of metal patterns would be embedded on eachsilicon wafer, the thinning step is most easily performed by reducingthe thickness of the wafer across the entire surface under which suchpatterns have been created. Doing so however, will inevitably weaken thewafer to the point where it cannot be manually handled. In such cases,unetched areas in the form of struts spanning the diameter of the waferare allowed to remain as strengthening members.

At this point, the x-ray mask is complete. By implementing these steps,a mask having blocking structures with lateral dimensions of less than 1micron are achievable. The mask is utilized by placing its planarizedsurface 32 directly onto the surface of the article which is to beexposed to the synchrotron radiation, and illuminating this assemblywith the radiation.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

What is claimed is:
 1. An x-ray mask tool, comprising: a siliconsubstrate having a thickness, a top surface, and a bottom surfacessubstantially parallel to said top surface, said silicon substratehaving at least one pattern etched into said top surface, said at leastone pattern extending to a depth of at least 10 microns into saidthickness and comprising a plurality of etched channels havingsubstantially parallel sides, said etched channels filled with a metaldeposit to provide a plurality of metal structures embedded into saidsilicon substrate extending to said depth and flush with said topsurface, said metal structures comprising features having lateraldimensions of less than 1 micron.
 2. The x-ray mask tool of claim 1,wherein said metal deposit is any metal capable of attenuating x-rayradiation to an energy of below about 0.1 KeV in a distance of less thanabout 100 microns.
 3. The x-ray mask tool of claim 1, wherein said metaldeposit is a metal selected from the group consisting of the Transitionseries of metals listed in New IUPAC Group Numbers 4-12 of the PeriodTable of elements, aluminum, tin, and alloys thereof.
 4. The x-ray masktool of claim 1, wherein said metal deposit consists essentially ofgold.
 5. The x-ray mask tool of claim 3, wherein said metal depositcomprises a thin vapor deposited first metal layer.
 6. The x-ray masktool of claim 5, wherein said vapor deposited first metal layercomprises a first layer of chromium.
 7. The x-ray mask tool of claim 5,wherein said metal deposit is deposited by electroplating.
 8. The x-raymask tool of claim 5, wherein said metal deposit is deposited byelectroless deposition.
 9. The x-ray mask tool of claim 5, wherein saidmetal deposit is deposited by thermal or particle vapor deposition. 10.The x-ray mask tool of claim 5, wherein said metal deposit is depositedby sputter deposition.
 11. The x-ray mask tool of claim 5, wherein saidmetal deposit is deposited by molecular beam epitaxy.
 12. The x-ray masktool of claim 1, wherein said silicon substrate is thinned from saidbottom surface to a thickness of less than 100 microns across a zoneencompassing said pattern.